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1,1,2,2-tetrachloropropane exists in two forms, trans and gauche, in both liquid and gaseous phases. The energy differences and the barrier to rotation between these two forms have been measured using n.m.r. and vibrational spectroscopy. There is fair agreement between the values of the energy difference (ET-EG) in the vapour obtained from direct i.-r. studies of the vapour (-lOOOrt 200 cal mol-') and from extrapolation of n.m.r. measurements made in solvents of different dielectric constant (-870 cal mol-I). There is poorer agreement between values for the barrier to rotation from n.m.r. (10.2+ 1 kcal mol-') and i.-r. (12.9+ 1.5 kcal mol-I), which is probably due to the fact that n.m.r. measures the rate of interconversion directly whereas the i.-r. method uses only a single fundamental torsional frequency in a potential energy minimum to determine the complete shape of the potential energy curve.